Friday, 9 July 2010

A walk with rocks in the Sierra de las Nieves

With the help of Google Earth, we cobbled together a stunning hike in the nearby Parque Natural Sierra de las Nieves. It is a walk with much to offer from a griffon vulture perching place (clap your hands just after the sun is up and out they flap) to chilling reminders of the bitterly fought Spanish Civil War, from perfect picnic places to views of distant cultures like Marbella! The walk wanders through pine forests and along easy trails.

Loff, a recent guest of Hotel Los Castanos, walked this trail and surprised us with his geological insights which I have his permission to share with you. Whilst I was watching the birds, looking out for processional caterpillars, and savouring the prospect of my boiled egg sandwich that I would enjoy when we got to the top, Loff was rock-spotting. And he kindly supplied photographs of each rock that he spotted so you too can recognise them when you walk the Sierras. Thus spake Loff:

This walk is the best one for seeing the main rock types of the area. Not only can you see peridotite (see previous blog for more info on that) but also the rocks that it had to penetrate as it rose, plus some of the metamorphic rocks that were created by its heat.

This rock will appear different colours depending on how much weathering it has experienced. Freshly broken peridotite looks dark in colour, often a deep green with a metallic lustre. If, however, it has been well weathered it will resemble a rusty coloured sandstone, often with a heavily pitted surface. This is due to oxidation of the iron contained within the peridotite. There is a lot of peridotite just lying on the track in many places around the walk.

3 types of Limestone
If it's light grey, it’s probably limestone. You might be lucky enough to find up to 3 types of limestone on this walk.

The first is called sparite, light grey and, when freshly broken, reveals clear or opaque shiny crystals. This rock can also display banding which is the appearance of tiny layers or stripes caused by slight changes in the environment during deposition.
The second is micrite which can be a slightly darker grey, or even blue, with a dull or matt texture on freshly broken surfaces. The crystals in this are so small they cannot be seen with the naked eye.
The third is dolomite which is usually a light brown and can have the texture of either of the above limestones. This is a limestone that has had its calcium replaced by magnesium. ( Note to Di: I didn’t find any of this but I believe it exists in the area – further research required!)

This can be found if you visit the gun emplacement as you will pass a working marble quarry. The marble has been formed by the metamorphosis of the limestone due to the heat generated when the peridotite intruded the crust. The brownish colour that can be seen in some of the quarry faces is the discolouration of the marble due to impurities like quartz in the original limestone.
Keep an eye open on the tracks you are walking for stones that are black and have a compact granular texture. The acid test for this rock, of course, is to see if it will deflect a compass needle. It’s fairly common in the area but not in vast quantities like the limestone. Its presence is down to the magmatic action of the rising peridotite.

Hornfels might be in the area
This is a metamorphic rock created by contact metamorphism when molten rock ( like the peridotite) comes into contact with something like a mudstone. The mudstone is completely re-crystalised into a rock which has crystals all roughly the same size, but their colours vary depending on the type of mineral, so the rock looks more like a mosaic.

And finally, it should be noted that granite has been found in splits in the rock in this area.

My thanks to you, Loff, for your painstaking recording of your finds and to Karen for transcribing the text. It is a great addition to the information we can offer our guests.

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